[blaisem]

In the above scheme, on the left side we can see electrons in the 2,3 aromatic bond being relocated onto the nitrogen on the right side. I understand electrons in an aromatic bond to be delocalized around the entire cycle. When the electrons are pushed onto nitrogen, I assume they are not localized as a lone pair, but still delocalized as an aromatic lone pair, similar to pyrrole.

So my question is: If those two electrons are part of the same delocalized aromatic pi cloud whether they are in a bond or as a lone pair, what is the difference between the heterocycle on the left and the heterocycle on the right? Specific to those two shifted electrons, what physical function changed in being drawn as a lone pair instead of a bond?

I'm also wondering whether the right heterocycle is aromatic at all, because I can't draw clean resonant structures within the ring due to the double bond on C2, even though it's Sp2 hybridized. On the other hand, examining the attached enolate, the keto-resonance structure I think would lead to a hypervalent sulfur; is there maybe a larger conjugated system between the heterocycle and the enolate?

Thanks for any advice!

The scheme is taken from the mechanism for the coenzyme "thiamine phosphate's" role in decarboxylation of pyruvic acid, on wikipedia.

[Babcock_Hall]

I may not be understanding your question completely.  However, it seems to me as if this is a case of simple resonance delocalization between the nitrogen atom of the thiazolium ring and the carbon which is exocyclic to the ring.